Safety workbenches, and particularly those for processing microbiological samples, as are described, for example, in DE 44 41 784 C2, protect from contamination by bioaerosols, which occur and are released in microbiological work. The contaminated air flow is continued as a directed air flow and conducted over filters, which hold back the contaminants from the air flow, with the aid of at least one fan within the safety workbenches.
Safety workbenches differ in their safety precautions and are constructed, tested, and licensed in accordance with the various international standards. Inter alia, safety workbenches offer personal protection or personal and product protection.
Safety workbenches which only offer personal protection are referred to as class I safety benches, the personal protection being achieved by suctioning outside air through the work opening into the working chamber of the safety workbench. As long as this outside air flow is not obstructed and sufficient air is suctioned in, particles and aerosols may not reach the outside from the inner chamber of the safety workbench. The suctioned external air thus forms an air curtain flowing through the work opening, which protects the person working at the safety workbench and/or the environment from contamination by the particles.
Adequate personal protection is a requirement for the operation of safety workbenches. This property of a safety workbench, also referred to as retention capability, is defined by a precisely established air entry velocity into the work opening, for example. It is directly proportional to the exhaust air flow, so that changes of the exhaust air flow have a direct influence on the personal protection and on the safety of the user.
Class II safety workbenches also offer, in addition to personal protection, protection to the work objects in the workbench from contamination from the outside or from contamination by other samples located in the workbench (so called cross-contamination). The protection from these types of contaminations is also referred to as product protection. The product protection results in that a part of the air flow suctioned into the workbench is fed to the inner chamber as a circulating air flow again after the filters. This circulating air flow is typically directed in a vertical falling flow from top to bottom in the working chamber of the workbench. This circulating air flow, which is also referred to as “downflow”, washes around the objects located on the work plate and prevents contaminated air from the outside or from other samples from coming into contact with these objects. The circulating air flow is in turn incident in the area of the intake opening, which is usually located on the front edge of the work plate, on the outside air flow flowing into the inner chamber, so that no particles may penetrate to the outside. The product protection, including the protection from cross-contamination, is thus decisively achieved by the relationship between downflow and an air entry velocity of the outside air flow.
To generate these air flows, a normal class II safety workbench has at least one fan. Separate fans are frequently provided for the circulating air flow and the exhaust air flow, which are referred to in the following as circulating air or exhaust air fans. The air suctioned from the working inner chamber is guided via filters, such as a circulating air filter and an exhaust air filter. These filters are high-performance suspended matter filters, such as HOSCH or HEPA filters, which are capable of filtering the relevant microorganisms out of the air flow.
Adequate function of the fans thus has great significance for the safety of the safety workbench. The function of the fans is therefore typically monitored automatically during operation of the safety workbench to be able to recognize malfunctions or even breakdowns in a timely manner. For the monitoring, the volume delivered by the fan (the air quantity) per unit of time and/or the flow velocity is typically measured directly or indirectly. One possibility for this purpose is the use of a calibrated anemometer. However, it is also possible to determine, instead of the flow velocity, a value representative thereof. This may be the pressure differential which exists between the intake side of the fan and its outlet side, for example. Two barometric cells or similar devices may be used for the measurement, one of which is situated in front of and one behind the fan. A setpoint value is stored in a control and/or regulating device of the safety workbench for the selected measured variable for each of the fans. This setpoint value is permanently predefined by the producer of the safety workbench. It is used during the operation of the safety workbench as a comparison value for the safe operation of the fan. In addition, deviation margins from this setpoint value are fixed and also stored at the factory. Safe operation of the fan is assumed within these margins. Outside the range, however, adequate personal and/or product protection may no longer be ensured. In the event of deviations from this range, a visual and/or acoustic alarm is therefore typically triggered, which is to indicate the unsafe operation of the safety workbench to the user. The deviation margins are therefore frequently also referred to as alarm limits. The alarm limits are fixed by legal guidelines in some countries. Examples of safety workbenches having a safety monitoring system which monitors the operating parameters of the safety workbench during working operation are described in EP 1609541 A2 and EP 1356873 A2 of the applicant.
Setpoint values for the fans and alarm limits are measured by the producer of the safety workbench in the factory either for every workbench or representatively on one or more safety workbenches as representatives of a specific type of workbench and stored in every safety workbench. This procedure has the disadvantage, however, that the location at which the setpoint values for the fans and the alarm limits are determined and stored in the safety workbench does not correspond with the location at which the safety workbench is to be put into operation and operated further. As a function of the barometric pressure existing at the particular installation location, other values would therefore result upon renewed measurement of the setpoint values and the alarm limits than were stored in the safety workbench at the factory. Different pressure conditions may also result as a function of whether or not the safety workbench is connected to a building exhaust system. In addition, the measurement devices, such as measurement sensors, which are used for ascertaining measured values to monitor the function of the safety workbench, may display a different measurement behavior, due to mechanical strain during the transport or for other reasons, than during the measurement performed at the factory. These factors typically have the result that the measured values ascertained at the factory no longer correspond to the measured values at the operating location of the safety workbench. As a result thereof, the alarm limits set at the factory are also shifted in relation to the actually desired limiting values, so that an alarm as a result of unsafe operation of the safety workbench is triggered either too early or too late.
To prevent these false alarms, safety workbenches are often recalibrated by a service technician after being installed at the desired working location, and the setpoint values and alarm limits stored at the factory are set again by hand. However, this procedure is complex, time-consuming, and costly. In some countries, a safety workbench is required to be installed and put into operation by a service technician. However, this is not true everywhere, and safety workbenches are frequently put into operation by a service technician without further measures and recalibrations. However, if the safety workbench is then operated outside the established setpoint values and defined alarm limits, this represents a significant safety risk.